Fm demodulator circuit for use in a facsimile system



Sept. 16, 152 .69 7 P. J CRANE 3,467,772

FM DEMODULATOR CIRCUIT FOR USE IN A FACSIMILE SYSTEM Filed March 18.1966 2 Sheets-Sheet 1 mar 7 a fiel c/ar f aa/ye #eadg' err Man/ 0/" &

IA/I EA TOR-I Sept. 16, 1969 'P. J. CRANE 3,467,772

FM DEMODULATOR CIRCUIT FOR USE IN A FACSIMILE SYSTEM Filed March 18,1966 2 Sheets-Sheet F. 02

15 r/c +18 /c HIV/5 *1 (i r/c United States Patent 3,467,772 FMDEMODULATOR CIRCUIT FOR USE IN A FACSIMILE SYSTEM Paul J. Crane,Torrance, Calif., assignor to The Magnavox Company, a corporation ofDelaware Filed Mar. 18, 1966, Ser. No. 535,442 Int. Cl. H04n 1/40 US.Cl. 1786 9 Claims ABSTRACT OF THE DISCLOSURE This invention relates to afacsimile system which converts an image into frequency modulatedsignals representing the image and transmits the signals to a receiverwhich demodulates the signals to recover the image. The system includesa demodulator which converts the frequency modulated signals to signalshaving substantially no duration in representation of white color andhaving a progressively increasing duration in progressively increasingshades of color toward black. The demodulator further hascharacteristics of producing a reference voltage for a white color andof producing progressive variations from the reference voltage forprogressive variations toward a black color. By providing thedemodulator with these characteristics, an accurate reproduction of theimage at the receiver is obtained.

This invention relates to facsimile transceivers of the sort describedin application Ser. No. 669,315 filed Sept. 20, 1967 on behalf of GlennA. Reese and Paul J. Crane entitled Facsimile Systems, which in turn isa continuation of application Ser. No. 549,759 (now abandoned) filedApr. 21, 1966 and entitled Facsimile Systems, which in turn is acontinuation of application Ser. No. 176,248 (now abandoned) filed Feb.28, 1962 and entitled Facsimile Systems, for transmitting the contentsof documents to remote locations using standard telephone transmissionfacilities; and, more particularly, there is disclosed herein afrequency-modulated signal demodulation system (FM demodulator) that isparticularly well adapted to provide optimum reception therein.

Briefly speaking, the function of a facsimile transceiver system of thesort referred to in application Ser. No. 669,315 is to scan documents ata transmit station and develop an electrical signal representative ofthe contents of the document. This electrical signal is then modulatedinto a form suitable for transmission over standard telephonetransmission lines. The preferred form of modulation for such base bandsignals is to frequency-modulate them at a low frequency in the audiorange transmittable by ordinary telephone circuitry, preferably in therange of 1500 c.p.s. to 2500 c.p.s.

The FM-modulated signal is then coupled into standard telephonetransmission lines and taken therefrom again at the receiving stationthrough the same standard handsets that are used for regular voicetransmission, so that no special jacks or other electrical hookups arerequired. With such an arrangement a facsimile transceiver can be usedas a portable or movable oflice appliance, and operation becomes simpleenough that the ofiice personnel using the transmitting and receivingtransceivers can simply dial one another in the conventional way andarrange for the transmissions. At the receiving station theabovementioned FM-modulated facsimile signal is demodulated to providean electrical signal that operates a printing 3,467,772 Patented Sept.16, 1969 device. The printing device then reproduces the contents of thedocument originally scanned at the transmit station. The purpose of theinstant invention is to provide an improved FM demodulator for use atthe receiving stanon.

The two great disadvantages of FM transmission in the facsimile systemdiscussed above are (1) the low frequency band width of the FM comparedto the highest modulation frequency (necessary in order to transmit overan audio system such as a telephone line) means that the fundamentalfrequency of the FM signals being demodulated by a receive-modetransceiver set is very low. Such low fundamental waveforms and FMrequire large and expensive filters in order to convert them intosufliciently smooth DC. One object of the instant invention is toeliminate the necessity for the expensive and complex filtering systemsheretofore used in the facsimile transceiver of the above-cited Ser. No.669,315 system.

Secondly, the FM system used in facsimile transmission of necessity hasone end of its band width representing white paper and the other end ofits band width representing black paper. At the white paper end, even aslight deviation in the DC output voltage of the FM demodulator willshift the white to gray, producing an off-shade ugly copy of thereceive-mode transceiver. Another object of the instant invention is toprovide an FM demodulation system wherein the accuracy of demodulationespecially at the low frequency end of the band width (corresponding towhite paper) is made more precise.

In the achievement of the above and other objects and as a feature ofthe instant invention there is provided a new PM demodulation systemwherein a first monostable multivibrator circuit and a second monostablemultivibrator circuit are operated in parallel, both beng triggered bythe same input FM signals. The time-constant circuitry resistances andcapacitances of the two parallel monostable multivibrators are so chosenthat the first monostable multivibrator has a very quick recovery time,while the second monostable multivibrator has just enough time torecover at the lowest frequency in the band with of the detector. Atfrequencies higher than this lowest frequency the second monostablemultivibrator is not able to recover fully. Therefore, by developing anoutput signal conditioned upon the difference of the timed states ofboth the first monostable multivibrator and the second monostablemultivibrator, a detector output signal may be developedthat has zeropower at white level and increasing power therebeyond.

As the frequency of the FM input signal increases, the recovery time ofthe first monostable multivibrator and the second monostablemultivibrator decreaes, giving the overall result that a larger outputsignal is produced by the detector as the frequency of the FM inputsignal goes up. This output signal consists of a train of pulses at afrequency corresponding to the frequencies of the input to the detector,but with a pulse width that is approximately proportional to the rise inthe input frequencies above the low end of the FM band width. Since theoutput pulses are of the same amplitude, the amount of power passedwithin their envelopes is proportional to their duty cycle-or, in otherwords, their durationand thus also rises approximately linearly from thezero-power frequency at the low end of the FM band width. The pulsesfrom the detector are subsequently filtered to provide a variable DCoutput signal. Since at the low end of the band width almost no power ispassed to the filter 3 by the detector, the heretofore difficult problemof high zero-level AC-carrier output voltage is avoided.

Other objects and features of this invention and a fuller understandingthereof may be had by referring to the following description and claimstaken in conjunction with the accompanying drawings in which:

FIGURE 1 is a block diagram of a facsimle transceiver in which thereadiness monitoring system of the instant invention is incorporated;

- FIGURE 2 is a schematic diagram of the circuit which is a preferredembodiment of the principles of this invention; and

FIGURE 3 is a graph of waveforms at certain points in FIGURE 2.

The system shown in FIGURE 1 represents a facsimile system which isdisclosed in detail in application, Ser. No. 669,315 filed Sept. 20,1967 in the name of Glenn A. Reese and Paul J. Crane entitled FacsimileSystems and Ser. No. 458,954 filed May 26, 1965 in the names of Rex J.Crookshanks and Glenn A. Reese, entitled Facsimile Transmission System.

Referring to FIGURE 1, the facsimile transceiver system upon which theinstant invention is an improvement has for its purpose the scanning ofan original and the reproduction of the contents of the original 10 uponcopy paper 12 at a remote location, using standard commercial telephonetransmission facilities (represented by the lines 14) to transmit thefacsimile signals. The system of FIG- URE 1 represents the most advancedpractice of the facsimile transmission art, according to which facsimilesignals may be sent and received through standard commercial telephonehandsets 16 when properly positioned upon acoustical coupling sets 18.In the transmit mode of a facsimile transceiver set (shown in thatportion of the block diagram in FIGURE 1 above the transmission lines14), the acoustical couplers 18 receive electrical signals from thetransceiver and convert them into acoustical signals whichare thencoupled into the handset 16. In the receive mode of a facsimiletransceiver (shown in block diagram below the transmission lines 14 inFIGURE 1) the transceiver acoustical coupler 18 receives acousticalsignals from the handset 16 and converts them into electrical signalsfor processing by the transceiver.

The scanning of the original 10 is accomplished by a pickup transducer21 mounted in such manner as to be rotated by an electrical motor 23,preferably of the hysteresis-synchronous variety, controlled by signalsfrom a power supply or motor drive amplifier 25 (hereinafter calledpower amplifier 24 for short) which derives its control signals from afrequency standard such as that provided by a turning fork 27 (or,alternatively, a crystalcontrolled oscillator or other highly accuratefrequency source). The electrical signals produced by the pickuptransducer 21 in response to the written matter on the original 10frequency modulate at 28. It is the practice to have white or blankspaces on the original show as unmodulated FM carrier (in one example,1500 c.p.s. carrier frequency) at the output of the modulator 28.

The frequency-modulated signals from the modulated 28 are then fed tothe acoustical coupler 18 which directs the signal into the telephonetransmission facilities 14 through the handset 16, to a receivinghandset 16'. Since standard commercial telephone are used at both endsin the system of FIGURE 1, either the sending or the receiving stationsmay initiate the transmission by dialing the other in the conventionalmanner.

The transmissions received by the handset 16' are transduced by thereceiving or reproducing set acoustical coupler 18 into electricalsignals which are fed to an equalizer circuit 32 which compensates forthe effects of transmission line distortion. The distortion-compensatedsignal from the equalizer 32 is fed to a limiting and filtering network34 which serves to reduce noise and signals other than the main FMcarrier which was originally transmitted. Thereafter, the signal isdemodulated by a detector system 35 of the sort to which the presentinvention is addressed. The detector 35 output pulses are fed to alow-pass filter 36 to be smoothed into a low-frequency or variable DCsignal which controls a printing transducer 38 which writes on the copypaper 12. The printing transducer 38 scans and prints in phase with thepickup transducer 21 due to rotation by another hysteresis synchronousmotor 23 powered from a power amplifier 25 deriving frequency signalsfrom a frequency standard 27 identical to one being used by the transmitmode.

As stated above, the FM signals sent by the facsimile transceiver systemof FIGURE 1 must be in a frequency range in which the telephone lines14, the handsets 16 and all their associated amplifiers and the like areresponsive. It has been the practice of facsimile transmission to use anFM band running from 1500 c.p.s., to 2500 c.p.s. with 1500 c.p.s.representing unmodulated carrier or lightest document tones (in otherwords, white), while the 2500 c.p.s. upper band limit represents darkestdocument tones (in other words, black). In the past, this 1500-2500c.p.s. FM band has required large and expensive filters to performdemodulation because of the low fundamental frequency of the detectedsignal to be filtered. Even worse, the filtering was least effective atthe lower end of the FM band, so that the very critical voltage levelrepresenting white paper received the least effective processing, withthe result that the prior FM demodulators in facsimile transmission hadto be very complex and very carefully adjusted in order to prevent aslight graying of the copy paper of the facsimiles produced by thereceive-mode transceiver. The principles of the instant invention asembodied in the circuit of FIGURE 2 lead to the production of a detectoroutput signal which has its output energy located chiefly in theharmonics of the fundamental frequency at the white level or low end ofthe FM band width, so that filtering is most effective at that point.Moreover, to further minimize the possibility of a slight gray signalbeing produced, the power passed from the FM detector is also minimaland almost non-existent at the low end of the FM band width.

The circuit shown schematically in FIGURE 2 has a positive power supply10, a negative power supply 12, a ground terminal 14, an input terminal16, a detector output signal terminal 18 and a carrier receptionindication signal terminal 19. In a model built according to the circuitof FIGURE 2, the positive power supply 10 was +18 volts DC, while thenegative power supply 12 was -18 volts DC.

The input signal applied at the terminal 16 takes the form shown at A inFIGURE 3 and is derived from the FM carrier arriving at the receive-modetransceiver by limiting and squaring the carrier waveform, phasesplitting the squared signal, differentiating each leading and trailingedge of the squared signal phases, and full-wave rectifying thenegative-going differentials of each phase to provide triggering pulsesat double the frequency of the input FM. This doubling of frequency hasthe advantage, of course, of getting a detector output frequency that isof higher frequency and thus more easily filterable. Moreover, theoverall detector or demodulator system 35 benefits from theabove-described processing in that the detector output frequency isthereby more widely separated from the highest frequency of theinformation signal imposed upon the input FM carrier.

The functions here required of a double monostable multivibrator circuitare provided by three transistors 20, 30 and 40. The general arrangementof the circuit is such that the transistor 20 is the normally-offtransistor in the monostable multivibrator arrangement, while thetransistors 30 and 40 are normally on. The transistor 40 cooperates withthe transistor 20 to form one monostable multivibrator circuit that hasa fast enough recovery rate to be able to perform or time uniformlythroughout the band width of input FM frequencies. On the other hand,the transistor 30 and its associated components cooperates with thetransistor 20 to form a partial monostable multivibrator circuit whichcan fully recover only at the lowest frequency in the FM band width.

The transistor 20 has an emitter 22, base 24 and collector 26. Theemitter 22 is directly connected to the positive power supply and isconnected through a biasing resistor 33 to the base 24. The base 24 isconnected through a resistor 29 to the input terminal 16, where thetriggering pulses shown in FIGURE 3A arrive in the circuit of FIGURE 2.Thus the triggering pulses 3A are applied through the resistor 29 to thebase 24 of the transistor 20. Because they are negative-going, theypulse the transistor 20 on.

The transistor 40 has emitter 42, base 44, and collector 46. The emitter42 is directly connected to the positive power supply 10 and isconnected through a biasing resistor 48 to the base 44. The base 44 isconnected through the series combination of a diode 50 and a resistor52. to the ground terminal 14. Because the emitter 42 is held at thevoltage of the positive power supply 10, while the voltage on the base44 tends to drop toward the voltage of the ground terminal 14, thetransistor 40 is normally in its saturated state in the operation of thecircuit of FIG- URE 2. However, the collector 26 of the transistor 20 isconnected through the series combination of a diode 54 and atime-constant capacitor 56 to the base 44 of the transistor 40. Thuswhen a trigger pulse at 16 switches the transistor 20 on, so that itscollector 26 suddenly goes from a voltage near that of ground line 14 toa voltage near that of the positive power supply 10, capacitor 56transfers this change so that the voltage on the base 44 is raisedabruptly to turn off the transistor 40.

A first plate 57 of the capacitor 56 is connected through a resistor 58to the negative power supply 12 and is connected through a diode 60 toground 14. When the transistor 40 is on and the transistor 20 is off,the plate 57 of the capacitor 56 is held very near the voltage of theground line 14 through its coupling thereto by the diode 60. A secondplate 62 of the capacitor 56 is connected to the ground line 14 throughresistor 52, but is held at a potential provided by the base emitterjunction (42-44) of transistor 40 and diode 50 which is near in voltageto the positive supply 10.

When the transistor 20 is switched on by a pulse applied at the inputterminal 16, the voltage on the plate 57 of the capacitor 56 suddenlyrises to very near the voltage of the positive power supply 10. Sincethe voltage across a capacitor cannot change instantaneously, this risein the voltage of the plate 57 causes the voltage on the plate 62 torise by a similar amount, so that the voltage on the plate 62 is morepositive than the positive power supply 10. Because the diode 50 blocksreverse current flow, the entire rise in voltage of the plate 62 is notcommunicated to the plate 44 of the transistor 40, but the plate 44 iselevated to a point where the transistor 40 turns off.

The efiect of the input difierential pulses shown in FIGURE 3A upon thetransistors 20 and 40 is shown by the waveforms of FIGURES 3B and 3C.FIGURE 3B shows the voltage on the collector 26 of the transistor 20,while FIGURE 3C shows the voltage on the collector 46 of the transistor40. It can be seen that when the transistor 40 is switched off in themanner described above, the voltage on its collector 46 drops from thatof the positive power supply 10 to a voltage very near that of theground terminal 14, due to the coupling of the collector 46 to ground 14through a diode 64. Since the collector 46 is connected through aresistor 66 to the base 24 of the transistor 20, the near-groundpotential thereof will be communicated to the base 24 to hold thetransistor 20 turned on, for the resistors 33 and 66 will then combinewith another resistor 68 connected between the collector 46 and thenegative power supply 12 to provide voltage division which ensures thatthe collector 46 is at ground potential and indeed would be below itwere not for the diode 64.

Immediately upon having its plate 57 raised to a high positive voltagedue to the switching on of the transistor 20, the capacitor 56 begins todischarge through the resistor 52 to the ground line 14. The relativevalues of the capacitor 56 ad the resistor 52 are chosen such that thisdischarge can take place in plenty of time before the arrival of anothertrigger pulse at the terminal 16. The discharge of the capacitor 56 hasthe eflect of lowering the potential between its first plate 57 and itssecond plate 62, so that the potential on the base 44 of the transistor40 begins to drop. When this potential finally goes low enough to switchthe transistor 40' into its conductive state, the collector 46 thereofis suddenly raised to the level of the positive power supply 10 and thetransistor 20 is turned off by the connection between the collector 46of the transistor 40 and the base 24 of the transistor 20. Thetransistor 40 then continues in its saturated state and the transistor20 continues its cut-olf state until another pulse arrives at 16.

The above discussion described the performance of the monostablemultivibrator centered about the transistors 20 and 40 in response toinput pulses at 16. According to the principles of the instantinvention, another partial monostable multivibrator circuit is formedabout the transistors 30 and 20, for the collector 26 of the transistor20 is connected through the series combination of a diode 70, acapacitor 72, and a diode 74 to the base 34 of the transistor 30. Thebase 34 being the control electrode of the transistor 30, the voltageson the collector 26 of the transistor 20 have a similar eltect on thetransistor 30 to the effect described in respect to the transistor 40.Just as the capacitor 56 discharges to the ground line 14 through theresistor 52, so the capacitor 72 discharges to the ground line 14through resistor 82. The capacitors 72 and 56 have substantially equalvalues, and resistors 52 and 82 have substantially equal values, whichconditions yield equal discharge times, provided that the initialvoltages on capacitors 72 and 56 are equal.

Similarly to the electrical connections of the transistor 40, thetransister 30 has a biasing resistor connected between its base 34 andits emitter 32.. Likewise, its base 34 is connected through the diode 74and a resistor 82 to the ground line 14, and a diode 84 similar to thediode 60 ensures that a first plate 86 of the capacitor 72 can neverdrop below ground potential. The voltages o the collector 36 of thetransistor 30 are shown as waveform D of FIGURE 3.

The detected output of the circuit of FIGURE 2 is applied to theterminal 18 through a transistor having emitter 102, base 104, andcollector 106. The emitter 102 is directly connected to the ground line14, while the collector 106 is connected through a resistor 108 to thenegative terminal 12. The base 104 is connected to the ground line 14through a diode 110 and is connected to the negative line 12 through aresistor 112. Since the base 104 is the control electrode of thetransistor 100, signals from the collector 36 of the transistor 30 arecoupled thereto through a resistor 114, while signals from the collector26 of the transistor 20 are also coupled thereto through a resistor 116.In actual circuits built according to the schematic of FIGURE 2, theresistors 114 and 116 were chosen as equal in value. The resistor 112was then chosen so as to be smaller in value than either the resistor114 or the resistor 116, but larger in value than the total resistanceof the parallel combination of the resistors 114 and 116. The result ofthis arrangement is that when either one of the transistors 20 or 30 ison while the other is off, voltage division between the positive powersupply 10 and the negative power supply 12 through the on transistor 20or 30, thence through either the resistor 114 or the resistor 116, andfinally through the resistor 112 to the negative line 106, will causethe base 104 of the transistor 100 to fall below the ground potential ofthe emitter 102, thus making the transistor 100 conductive. For similarreasons,

the base of the transistor 100 will receive a negative potential andcause the transistor 100 to become conductive when both of thetransistors and 30 are non-conductive. When the transistor 100 isconductive, the collector of the transistor 100 and accordingly theoutput terminal 18 are at substantially ground potential. On the otherhand, if both the transistor 20 and the transistor 30 are on(conductive) the voltage on the base 104 will be above ground potentialso that the transistor 100 is switched to a non-conductive state,thereby causing the collector of the transistor 100 to be at a negativepotential of approximately -18 volts. In this manner the outputwaveforms B and D of the transistors 20 and 30 are essentially AND gatedto provide the waveforms E On the terminal 18 to the output of thedetector.

An examination of FIGURE 3E will show that the waveform shown there hasexactly the characteristics desirable for facsimile transmission orindeed for any FM transmission at low frequencies. At the outset, itshould be noted that the transient spikes 121 shown in FIG- URE 3B arenot purposely generated there or indeed at all useful in the demodulatioprocess contemplated by the invention. They occur due to the differencein switch-on and switch-off time of most transistors. Since they areboth small in power content and steep in both rise and fall, they areeasily filtered off and are insignificant in the final demodulatedoutput of the system using the detector of FIGURE 2. The significantpulses in FIGURE 3 are those shown at 122.

Inasmuch as no proportional voltage signal is produced on the outputtreminal 18 by the circuit of FIGURE 2 when the input frequency is belowthe detector band width, it is necessary to demodulate the receivesignal in a more conventional fashion if it is desired to have anindication whether FM carrier is being received in the circuit whereby avoltage indication would be present even when a lower than normalfrequency signal or no signal is being received. To perform thisfunction there is provided a transistor 120 having emitter 123, base124, and collector 126. The collector 126 of the transistor 120 is theoutput electrode thereof and is directly connected to the terminal 19,while being also connected through a resistor 128 to the positive powersupply 10. The emitter 123 of the transistor 120 is directly connectedto the ground line 14.

The base 124 of the transistor 120 is the control electrode thereof andis coupled through two resistors 130 and 132 to the collector 46 of thetransistor 40. A capacitor 134 is coupled from a point between theresistors 130 and 132 to the ground line 14, while two resistors 136 and138 are connected from the same point between the resistors 130 and 132to the negative power supply 12.

The circuit centered about the transistor 120 has the function in thefacsimile system of providing an indication that FM carrier is beingreceived at the receive-mode transceiver. The effect of thiscarrier-reception indication is described in more detail in applicationSer. No. 537,177 filed Mar. 24, 1966, on behalf of Paul J. Craneentitled Readiness Monitoring System which i turn is acontinuation-in-part of application Ser. No. 488,459 (now abandoned)filed Sept. 20, 1965 and entitled Readiness Monitoring System. Brieflyspeaking, the indication that carrier is being received is used toprovide a signal that is applied at the terminal 156 of FIGURE 4 ofapplication Ser. No. 537,177. The presence of a carrierreception signalat 156 is necessary in order to activate the motor 22 of thereceive-mode transceiver.

Therefore, the circuit centered about the transistor 120 is adapted toprovide a signal indicative of whether or not the frequencies receivedat 16 are above or below the frequency band of the FM carrier to bereceived from the transmit-mode transceiver. The exact criticalfrequency of the circuit may be adjusted by varying the value of theresistor 138. In effect, the signal on the collector 46 of themonostable multivibrator transistor 40 is processed in the conventionalform heretofore used to provide the indication on the terminal 19. Thusthe capacitor 134 is chosen of such value as to provide lowpassfiltering of the waveform 3C, with the result being a smooth variable DCvoltage signal as represented by the waveform G of FIGURE 3. Themagnitude of the voltage of the waveform G is approximately proportionalto the frequency of pulses received at 16 all the way down to zerofrequency.

The operation of the above-described video detector circuit is asfollows. Referring to FIGURE 3, input pulses derived by the limiting anddifferentiation of FM carrier received by the receive-mode transceiverappear at the input terminal 16. These pulses are negative-going andthus cause the base 24 of the normally-off transistor 20 to go negativeof the emitter 22, whereupon the transistor 20 switches on. FIGURE 3Bshows the result of this switching on the collector 26 of the transistor20. When the transistor 20 is off, the collector 26 drops to a voltagelevel near that of the ground line 14. However, the moment thetransistor 20 is switched on, its collector 26 is almost directlyconnected to the positive power supply 10, thus causing a steep rise 200at the leading edge of each pulse shown in FIGURE 3B. Trailing edges 202of the FIGURE B pulses occur when the transistor 40 switches on againafter having been held off while the transistor 20 was on.

The transistor 40 is normally held in its saturated state because whileits directly coupled emitter 42 is always at the potential of thepositive power supply 10, its base 44 is below that potential because ofthe voltage division between the power supply 10 and the ground line 14through the resistors 48 and 52. When the transistor 20 switches on, thehigh voltage levels 204 of the pulses 3B are coupled through thecapacitor 56 and diode 50 to the base 44, causing its current input tobe removed. Thus the transistor 40 is switched off when the transistor20 switches on. The effect upon its collector 46 is shown by the leadingedge 210 of the waveform of FIGURE 3C. The duration of the waveform ofFIGURE 3C is nearly the same as the duration of the waveform B, becauseboth transistors 20 and 40 are switched at nearly the same time whendischarged of the capacitor 56 permits the base 44 to drop below thevoltage of the emitter 42 once again. Thereupon the voltage on thecollector 46 is returned to the positive supply 10 shown at 212. Sincethe collector 46 is coupled to the base 24 of the transistor 20, thetransistor 20 is cut off nearly simultaneously because its base 24 is atthe same voltage as its emitter 22.

The waveform B of FIGURE 3 is also coupled to the base 34 to cut off thetransistor 30. The transistor 30 is normally on, as is the transistor40, and for roughly the same reasonbecause while its emitter 32 isdirectly coupled to the positive power supply its base 34 is held at apotential below that of the positive power supply due to the voltagedivision between the positive power line 10 and ground 14 through theresistors and 82. The steep rise 200 of the waveform B, however, causesthe collector voltage of the transistor 30 (shown as the waveform D ofFIGURE 3) to drop because the transistor 30 is no longer switched on tothe positive power supply line 10. The drop 216 goes to a lower voltagelevel 218 very near that of the ground line 14. Since the waveform D andthe waveform B are effectively summed in a logic sense to provide thewaveform E of FIGURE 3, the slow rise of the leading edge 216 ascontrasted with the fast rise of the leading edge 200 leaves a slightlymomentary difference which results in the transient that was discussedabove.

When the pulses indicated at A have a relatively low frequency, thecapacitor 72 is able to be recharged through a circuit including theresistor 76 and the rheostat 78 after the fall 202, but before the nexttrigger pulse A. This causes the operation of the transistor 30 tocorrespond substantially to the operation of the transistors 20 and 40so that the transistor 100 never becomes nonconductive and no pulses 222are produced, except for the pulse produced as a result of the slow riseof the leading edge 216. However, when the frequency of the pulsesindicated at A in FIGURE exceeds a critical value dependent upon thesetting up of the rheostat 78, the capacitor 72 cannot become fullyrecharged before the arrival of the next pulse A. This will cause theleft plate of the capacitor 72 to be less positive after transistor 20switches on and will then allow transistor 30 to switch back on soonerthan if capacitor 72 had been fully recharged. The period ofnon-conductivity of the transistor 30 decreases as the frequency of thepulses A increases since the high RC constant of the capacitor 72 andthe resistances 76 and 78 becomes increasingly ineffective in providingcapacitor 72 with a sufficiently high voltage charge to render thetransistor 30 non-conductive during all of the time that the transistor20' is conductive.

According to a main principle of the instant invention, when recovery ofcapacitor 72 is not complete, the trailing edges 219 of the pulses ofFIGURE 3D occur a short interval before the trailing edges 202, for theinterval of time between each trailing edge 219 and each trailing edge202 is a period when the voltage on the base 104 of the transistor 100is determined by voltage division through the resistors 114 and 116 andthe resistor 112. Because the resistors 114 and 116 in parallel arelower in value than the resistor 112, the base 104 rises above thevoltage level of the emitter 102 which is coupled directly to ground114. As a result, the transistor 100 is cut off for this period so thatthe voltage on its collector 106 drops to that of the negative powersupply 12. This results in the pulse 122 which is the main and mostimportant output of the circuit of FIGURE 2.

In this way, the duration of the pulse 122 is dependent upon thefrequency of the pulses A in FIGURE 3 above a critical value such as3000 pulses per second. Since a pulse rate of 3000 p.p.s. (pulses persecond) corresponds to a white image, the pulses 122 are not providedwith any Width for a white image so that a ground potential is producedat the terminal 18. The production of a ground potential as a referenceis important in stabilizing the operation of the system and in providinga reference for the proper production principally of white, but also ofdifferent shades of black. As the image progresses in different shadestoward a completely black tone, the pulse rate of the pulses A increasesand the duration of the pulses 122 correspondingly increases so that thedirect potential average on the terminal 18 correspondingly becomesincreasingly negative. This provides a reliable output on the terminal18 of the characteristics of the image to be reproduced at each instantafter it is passed through a filter that rejects frequencies of 3000cycles per second and frequencies above 3000 cycles per second. Thisoutput is provided in a straightforward manner so as to simplify videofiltering. It can be seen from examination of the waveforms B and D thatthe pulse 122 occurs only at those instants when both waveforms B and Dare at their higher voltage levels, 204 for the waveform B and 226 forthe waveform D. In this situation, the resistors 114 and 116 areessentially splitting the voltage division current which also passesthrough the resistor 112. The selection of the three resistors 112, 114and 116 is such that, when the resistors 114 and 116 are acting inparallel this way, the voltage on the base 104 is above ground potentialso that the transistor 100 is cut off and a negative potential isproduced at the terminal 18. In all other time periods represented inFIGURE 3, one of the waveforms B or D is at a high level while the otheris at a low level, so that voltage division occurs only through one ofthe resistors 114 and 116, operating in series with the resistor 112. Inthis situation the transistor 100 will be turned on.

In addition to the functions described above, the diode 54 performscertain additional functions of some importance. For example, the diode54 operates to isolate the resistor 27 from the capacitor 56 so as toprevent the recharge time of the capacitor 56 from increasing the timethat the transistor 20 is switching from a conductive to anon-conductive state. This is important in ensuring that the transistoris switched rapidly from a nonconductive state to a conductive state inaccordance with the instantaneous switching of the transistor 20 from aconductive state to a non-conductive state. The diode 84 provides asimilar function in isolating the resistor 76 and the rheostat 78 fromthe capacitor 72 during the switching of the transistors but is alsoimportant in allowing the capacitor 72 to be recharged at a ratecontrolled by the resistor 76 and the rheostat 78 and independently ofany other controls.

Referring to FIGURE 3F, the waveform shown there represents the voltageon the plate 57 of the capacitor 56 in the course of the operation ofthe circuit of FIG- URE 2. As long as the waveform B is at its positivelevel 204 the waveform F is at a similar positive voltage 230 becausethe connection between the point B and the point F is through a diode54. When the high voltage level 204 terminates at 202, however, the highlevel 230 of the waveform F declines along the slope 232 represented bythe discharge of the capacitor 56 through the resistor 58. The discharge232 ends when the plate 57 of the capacitor 56 reaches the voltage levelof ground line 14 at 234. This voltage level 234 is, of course,determined by the drop below the ground level 14 across the junction ofthe diode 60 and thus is at essentially ground level. It should be notedthat while the plate 57 is at the level 234 very near ground, the plate62 of the capacitor 56 is at a higher voltage level caused by the diodeaction of the base emitter junction of transistor 40 connected to powersupply 10. Thus when the level of the plate 57 jumps back to the highervoltage level 230, as represented by the leading edge 236, the voltageacross the capacitors 56 is high enough to cause the voltage on theplate 62 to go to a level above the voltage at the power supply 10 untilsuch time as the capacitor 56 is able to discharge through the resistor52.

As was stated above, the performance of the capacitor 72 is essentiallythe same as that of the capacitor 56 except that the capacitor 72 doesnot have as steep a recharge curve 232 as does the capacitor 56. This isbecause the value of the resistors 76 and 78 is far higher than thevalue of the resistor 58. For example, in a circuit built according tothe instant invention the value of the resistor 58 was 16K ohms, whilethe value of the resistor 76 Was 182K ohms and the value of the resistor78 was adjustable to as high as 50K ohms. The result is that while thecapacitor 56 is able to recharge to its fully charge level 234 aftertransistor 20 turns off, but long before the next trigger pulse Aarrives, the capacitor 72 cannot fully recharge between trigger pulsesunless those trigger pulses are arriving at the lowest frequency to behandled by the detector-in the facsimile transmission system discussedherein, 3000 p.p.s. (the 1500 c.p.s. lower limit of the FM band, afterdoubling by full wave rectification).

To the extent that the capacitor 72 is not able to fully recharge, itwill not reach that voltage level that it would normally reach if itwere fully recharged. Because of the values of the resistors 52 and 82are the same, this will mean that the capacitor 72 will be able todischarge from its high positive voltage down to the emitter voltage oftransistors 30 and 40 more quickly than the capacitor 56. It isessentially this difference in discharge time that results ultimately inthe pulses 122, for the difference in discharge time creates adifference in switching times between the transistor 30 and thetransistor 40 (and therefor transistor 20) and thus a difference in timepositioning between the trailing edge 219 and the trailing edge 202.

If the variable resistor 78 is properly adjusted, the pulse 122 will benon-existent or a mere transient that will filter out easily at thelower end of the FM carrier band width which, as mentioned above,represents white level or unmodulated FM carrier. In the smoothing orlow-pass filtering circuitry which follows the circuit of FIGURE 2 andis connected to the terminal 18, the transient representing unmodulatedFM would easily filter out because it is composed almost entirely ofhigh frequency components and has almost no power included in itsenvelope.

On the other hand, as the waveforms A arrive at greater frequencies, sothat the time duration between them is less, the capacitor 72 will notbe able to fully recharge to the same voltage level as the level 234 ofthe capacitor 56. The result is that the transistor 30 will be able toswitch on faster than the transistor 40 upon the introduction of eachinput pulse. This will mean that the trailing edge 219 will appearprogressively farther forward of the trailing edge 202 in time as thetrigger pulses of FIGURE 3A arrive at higher frequencies. The result atthe terminal 18 of this relationship will be that the pulses 122 will belonger, of higher power content, or higher average negative voltage, andconstituted of more low frequency components as the input frequency ofthe FM signal to be demodulated rises from its low white level towardits high black level. As the power and low frequency component contentof the pulses 122 rises, and their average negative value rises, the DCoutput of the filtering networks connected to the terminal 18 will rise,giving very close reproduction of the original electrical signalsproduced by the pickup transducer 20 in the transmit-mode transceiver asdiscussed in connection with FIGURE 1.

Thus the circuit of FIGURE 2 achieves a detector signal appearing on thecollector 106 of the transistor 100 which has a frequency correspondingto the input trigger pulse and has a width that is very closelyproportional in a linear relationship with the increment in frequencyabove the frequency of the lowest carrier frequency in the FM band. Thisis an important accomplishment, for prior FM detectors normally had alarge component even at the minimum carrier frequency; and even if thisundesirable component were removed, the proportion of output was relatednot to the minimum carrier frequency, but to zero frequency, both ofwhich are undesirable characteristics for accurate determination ofwhite DC level (in an FM facsimile system where minimum frequencyrepresents white) which become increasingly acute when the FM carrier isin the audio frequency range rather than being in the higher rangesnormally used.

It can be seen from FIGURE 3E that the invention has gotten around theenormous filtering problems arising with prior audio-range FMdemodulators, whereby the lowest frequency components and high powercontent were present in the detector output at the low end of the FMband. By reversal of roles accomplished essentially by subtracting thewaveform B from the waveform D, applicant has achieved the moredesirable condition of having the lower power content and the lowestproportion of low frequency components present in the output pulse 122at the low end of the FM band. The inventive principle embodied in thecircuit of FIGURE 2 might thus be broadly stated as the subtraction froma normal monostable multivibrator detector pulse of a second pulse whichis width-varied as a function of the frequency of the FM signal to bedemodulated.

An actual circuit built and operated according to the schematic ofFIGURE 2 used the following components 120 s1054 (Fairchild).

Diodes:

Resistors (in ohms):

76 182K. 78 50K pot 80 3,300. 82 K.

136 100K. 138 50K pot.

Capacitors (microfarads):

The input pulses applied at 16 had a rate of 3600 pulses per second andan amplitude of 4 volts, while the output pulses 122 had an amplitude ofabout 18 volts.

Thus applicant has achieved an improved detector for FM demodulationsystems operating at low FM frequencies wherein a first monostablevibrator circuit and a second partial monostable vibrator circuit areeffectively operated in parallel, both being triggered by the same inputFM signals. However, according to the principles of the invention, thetime-constant circuitry, resistances and capacitances of the twoparallel monostable vibrators are chose differently, to the end that thefirst monostable multivibrator has a very quick recovery time while thesecond monostable multivibrator has just enough time to recover at thelowest frequency in the band width of the detector. Thereafter, atfrequencies higher than the lowest frequency in the band width of thedetector, the second monostable multivibrator is not able to recoverfully and yields a shorter monostable period.

The result of the above circuit arrangement is that by summing,comparing, or similarly sensing and processing the output signals of thefirst monostable multivibrator and the second monostable multivibrator,it is possible to develop a detector output signal conditioned upon thesimultaneous and coincident on state of both the first and secondmonostable multivibrators. Such an output signal will have its lowestpower content and its highest frequency power component composition atthe low end of the FM band being demodulated, while as the frequency ofthe FM input signal increases, the length of time when the firstmonostable and the second monostable are both on decreases, giving anoverall result that a larger output signal is produced by the detectoras the frequency of the FM signal goes up. Since at the low end of theFM band Width almost no power is passed and moreover the output pulsesare constituted mostly of high frequency components and closelyreferenced to ground, applicants low fre 13 quency detector has overcomethe problems to which it was addressed: the difiicult filtering problemand the problem of precise accuracy of DC output current level of theoverall FM demodulator system shown at 35 in FIGURE 1. 7

Although the invention has been described in its preferred form with acertain degree of particularity, it is understood that the presentdisclosure of the preferred form has been made only by way of exampleand that numerous changes in the details of construction and thecombination and arrangements of parts may be resorted to withoutdeparting from the spirit and scope of the invention as hereinafterclaimed.

I claim as my invention:

1. in combination in a facsimile transmission system for producing on apiece of copy paper a copy of an original document at a distancetherefrom utilizing standard commercial telephone transmissionfacilities:

first holding means for holding the original documens;

reading means operatively associated with the first holding means forproducing an electrical signal representative of the contents of theoriginal document;

frequency modulating means electrically connected to the reading meansfor frequency-modulating the electrical signal from the reading means;sending means electrically connected to the frequencymodulating meansfor passing the frequency-modulated signal output thereof into thestandard commercial telephone transmission facilities;

receiving means remotely located from the sending means for receivingfrequency-modulated signals from the standard commercial telephonetransmission facilities;

demodulating means electrically connected to the receiving means fordemodulating the frequency-modulated electrical signals from thestandard commercial telephone transmission facilities, said demodulatingmeans being characterized in that a detector circuit is used thereinwhich produces pulses of increased power content as thefrequency-modulated electrical signals increase in frequency;

second holding means for holding the copy paper on which the contents ofthe original document are to be reproduced; and

reproducing means operatively associated with the second holding meansand electrically connected to the demodulating means for reproducing thecontents of the original document in response to the output of thefrequency-modulated signal demodulating means.

2. The facsimile transmission system of claim 1 with the addition thatsaid demodulating means produces pulses of uniform height but addedlength as the frequency of said frequency-modulated signals increases.

3. The facsimile transmission system of claim 1 wherein:

the frequency-modulated signal produced by the frequency-modulatingmeans has an FM bandwidth and has the lowest frequency in the FMbandwidth representing the white or unmarked portions of the original;and

the detector circuit in said demodulating means produces pulses oflowest power and highest constituent harmonic content at said lowestfrequency.

4. The facsimile transmission system of claim 1 wherein the detectorincludes two monostable multivibrators, one of which has afrequency-variable output pulse length.

'5. In a facsimile system wherein frequency modulated signals areproduced at a transmitter to represent an image at each instant inaccordance with the frequency of the modulated signals at each instantand wherein the frequency-modulated signals are transmitted and whereinthe image is provided at each position on the image with a variableamount of white and black colors to define the image, a receiver forreceiving the trans- 14 mitted signals and for producing the image on amedium, including:

first means for receiving the frequency-modulated signals which aretransmitted, second means operatively coupled to the first means forconverting the frequency-modulated signals to signals havingsubstantially no duration in representation of a white color and havinga progressively increasing duration in progressively increasing shadesof color toward black, third means operatively coupled to the secondmeans for producing a reference voltage for a white color and forproducing progressive variations from the reference voltage forprogressive variations toward a black color, and

fourth mean operatively coupled to the third means for reproducing theimage on the medium in accordance with the voltage produced by the thirdmeans at each instant.

6. The receiver set forth in claim 5 wherein:

the second means includes a first stage having a relatively fastresponse time greater than that required to respond to the frequenciesof all of the frequencymodulated signals received by the first means andincludes a second stage having a relatively slow response time incomparison to the response time of the first stage and less than thefrequencies of at least some of the frequency-modulated signals receivedby the first means and includes a third stage responsive to thedifference in the response of the first and second stages for convertingthe frequencymodulated signals to signals having no duration inrepresentation of a white color and having a progressively increasingduration in progressively increasing shades of color toward black.

7. The receiver set forth in claim 5' wherein the first stage includes afirst resistor and a first capacitor for providing the relatively fastresponse time and wherein the second stage includes a second resistorand a second capacitor for providing the relatively slow response time.

8. In a facsimile system wherein frequency-modulated signals areproduced at a transmitter to represent an image at each instant inaccordance with the frequency of the modulated signals at each instantand wherein the frequency-modulated signals are transmitted and whereinthe image is provided at each position with a variable amount of whiteand black color to define the image, a receiver for receiving thetransmitted signals and for reproducing the image on a medium,including:

first means for receiving the frequency-modulated signals which aretransmitted,

second means having a first response time faster than the duration ofany of the received signals to provide first signals of referenceduration in accordance with the reception of the transmitted signals,

third means having a response time slower than the duration of thereceived signals for operating upon the received signals to providesecond signals having a progressively increasing duration in accordancewith progressive increases in the frequency of the received signals andhaving the reference duration for a white color,

fourth means operatively coupled to the second and third means forproviding a third signal having a duration dependent upon the differencein duration of the first and second signals and having no duration for awhite color,

fifth means operatively coupled to the fourth means for producing areference voltage for a white color and a voltage progressively varyingfrom the refer ence voltage for progressive durations of the thirdsignal, and

sixth means operatively coupled to the fifth means for 15 16 producing acolor at each instant in accordance with OTHER REFERENCES the voltagefrorn'the fifth means at that instant. 9. The receiver set forth inclaim 8 wherein the second Radlo'Facslmlle by sub'camer FrequencyModulameans includes a first resistor and a first capacitor with nOnMathes and Whitaker pamphlet a relatively short time-constant and thethird means in- 5 cludes a second resistor and a second capacitor with aRICHARD MURRAY Pnmary hxammer relatively long time-constant. R. L.RICHARDSON, Assistant Examiner References Cited US. Cl. X.R. UNITEDSTATES PATENTS 10 7 7,1, 73; 179 2; 329.410

3,296,539 1/1967 Felix 329--104 UNITED STATES PATENT OFFICE CERTIFICATEOF CORRECTION Patent No. 3.467.772 Dated September 16, 1969 Inven PAULJ. CRANE It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

Please correct the drawing in Figure 2 to show element 64 as a diode andto show that there is no connection between resistor 116 and thejunction of resistors 76 and 78.

Signed and sealed this 25th day of January 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ERT OTTSCHALK Attesting Officer Commissioner ofPatents

